Methods and devices of this type operate according to the so-called rivet dome welding principle, according to which firstly a connecting element which is fitted on one of the objects and has a projection formed thereon, the so-called rivet dome, is pushed through an opening of the other object and is then provided on the free end of the rivet dome with a moulded-on rivet head. The connecting element can also comprise a separate component which is pushed through both objects and has at one end an already previously applied head.
Moulding-on of the rivet head is effected by applying the described welding principle in that the rivet dome is plasticised by heating and, by means of of a tool which is sub-sequently described as a calotte or spherical cap and is pressed against the rivet dome, is brought into the form desired in the individual case. Generally at least one of the two assembly partners consists of a plasticisable plastic material although in principle it does however also suffice if solely the involved connecting elements or the rivet domes provided thereon are made of a plasticisable material.
Various methods are known for heating and plasticising rivet domes. For example ultrasonic welding is widespread (e.g. DE 100 46 451 A1). Ultrasonic vibrating tools, so-called sonotrodes, are hereby placed on the rivet domes in order to set the latter in high frequency vibrations and consequently to heat them in a very short time to their melting temperature. What is disadvantageous about this method is the high technical outlay and the fact that objects made of plastic material and metal cannot be connected together or only under certain conditions. The latter applies in particular when it concerns connecting objects which are sensitive to vibrations, as applies for example to high pitch loudspeakers which are intended to be incorporated in door linings of automotive vehicles.
In addition, methods and devices of the initially described types are known in which softening of the rivet domes is effected with the help of a previously heated cap which is brought into contact with the rivet domes. One problem resides hereby in that the heated cap, which should have as great a mass as possible in order to achieve high plasticising speeds, cannot be withdrawn from the rivet heads immediately after formation thereof. Rather, one must wait until the cap is cooled to a temperature which is significantly below the melting temperature of the rivet dome material since otherwise the danger exists that the rivet heads tear or the cap pulls threads. As a result, long welding times arise which can only be reduced to a tolerable amount by additional cooling devices.
Finally, methods and devices for contact-free transfer of heat to the rivet domes are known (e.g. EP 0 965 400 A2, DE 100 38 158 A1). Heated air is blown here against the rivet domes and/or the rivet domes are plasticised by electromagnetic radiation. A substantial disadvantage of this technique resides in the fact that the environment is also heated by the means provided for the heating and it is therefore difficult to control the process as a whole reproducibly. In addition, this is a reason why the heating times cannot be shortened arbitrarily.